8.1.4 Differential Manchester Coding

The 802.5 standard specifies that Differential Manchester coding is used for transmitting data on the ring. With this encoding technique, every bit is comprised of a half-bit time signal at a low or high polarity and other half-bit time signal at the opposite polarity. The mid-bit transition is for clocking only. The direction of the signals voltage transition will change whenever a ″0″ bit is transmitted and will stay the same for a ″1″ bit transmission. Therefore, each bit consists of two half-bits at opposite polarity. This scheme is said to be DC balanced. Figure 80 shows the principles of the Differential Manchester coding.

Figure 80. Differential Manchester Coding

To understand the reasons for DC balanced signalling, consider a baseband transmission using a simple transmission code such as NRZ (Non-Return to Zero) where during the transmission a ″1″ bit is represented as a + voltage and a ″0″ bit is represented as a - voltage (opposite direction of the ″1″ bit). Depending on the bit stream that is being transmitted with NRZ, it is possible that the line may spend on average more time in one voltage level than the other. In other words, the signal would be electrically unbalanced. Since the physical wire has capacitance, the unbalanced signal causes the wire to develop a constant DC voltage across the transmission media, which in turn, causes the transmission media to distort the signal. This phenomenon is called baseline wanderand the effect of that is to increase the interference caused by one pulse with a subsequent pulse on the transmission media. The result of this interference is that the signal is smearedso that the end of one bit, or a group of bits, overlap with the start of the next. This makes an unbalanced coding technique such as NRZ or NRZI unsuitable for high speed transmissions such as 4 or 16 Mbps token-ring networks. The use of Differential Manchester coding technique in token-ring networks would allow us to overcome this problem and have a DC balanced transmission on the media.

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IBM 8260 manual Differential Manchester Coding

8260 specifications

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